Circular machines for knitting tubular articles such as socks, stockings, tights and the like generally include an antitwist mechanism for ensuring that the article--which rotates with the cylinder during its manufacture and is generally kept in tension by a stream of air generated by a vacuum source--does not become twisted by coming into contact with nonrotating parts of the machine. These mechanisms generally comprise an at least partly perforated tubular element situated directly underneath and coaxial with the cylinder, with which it rotates, and situated inside a tube which is coaxial with it but fixed; said fixed tube possesses a side air connection. During the knitting of the sock or stocking, an arrangement of valves and suitable sealing systems between the perforated element and said outer tube keeps a vacuum source in communication with said side connection, in such a way that a stream of air comes from the needle cylinder and passes centrifugally through the perforations of said tubular element. In this way the stream of sucked air, which in the cylinder keeps the fabric taut, holds the article inside said rotating perforated element as the fabric is formed, so preventing it from becoming twisted. When knitting is completed, said arrangement of valves is used to close the side connection and open the communication between said vacuum source and an axial connection located at the base of the mechanism, in order to remove the article axially from the perforated tubular element and discharge it from the machine.
It frequently happens during fabric manufacture that the processing of the article is interrupted; the causes are numerous and have to do with the yarns used or with possible mechanical trouble. When such a situation arises, action must be taken to restart the production cycle and prepare the machine to commence a new cycle. This operation involves expelling the interrupted article and causes a shower of unknitted strands or filaments (a "reject" as it is known). During this operation one or more of these unknitted strands may enter one of the holes of the partly perforated tubular element. This will create a base for other strands to become caught, so encouraging the formation of a significant obstruction that could even appreciably reduce the cross section of the perforated tube. This can also happen on a machine with the cylinder not rotating. This phenomenon causes considerable problems, in particular:
1) the passage of the article through the perforated tube is inhibited or made difficult, sometimes preventing its expulsion; PA1 2) the optimum rate of flow of air that keeps the fabric of the next article taut is significantly reduced, lowering its quality; PA1 3) during the expulsion there is catching of the fibres of the article on the partial obstructions, which lowers the quality of the fabric (so-called "pulled filaments" occur).
Another common undesirable phenomenon is as follows: during the knitting of the article the perforations of the tubular element are traversed by the stream of air which keeps the article in tension; this stream also entrains fine waste matter from the yarn used in the manufacture of the article, particles of oil, dust and other impurities. These materials tend at least partly to adhere to the holes of the perforations and, over time, cause clogging, rendering the operation of the machine progressively more irregular. Periodical manual cleaning with dismantling of at least part of the device is therefore necessary.